msvcrt: fix NULL-pointer dereference

[Kirill K. Smirnov]

This fixes mingw32-windres.exe
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diff --git a/dlls/msvcrt/process.c b/dlls/msvcrt/process.c
index 9dbc43e..664b11a 100644
--- a/dlls/msvcrt/process.c
+++ b/dlls/msvcrt/process.c
@@ -142,12 +142,12 @@ static MSVCRT_intptr_t msvcrt_spawn(int flags, const MSVCRT_wchar_t* exe, MSVCRT
     return -1;
   }
 
-  msvcrt_search_executable(exe, fullname, use_path);
+  if (exe != NULL) msvcrt_search_executable(exe, fullname, use_path);
 
   memset(&si, 0, sizeof(si));
   si.cb = sizeof(si);
   msvcrt_create_io_inherit_block(&si.cbReserved2, &si.lpReserved2);
-  if (!CreateProcessW(fullname, cmdline, NULL, NULL, TRUE,
+  if (!CreateProcessW(exe == NULL ? NULL : fullname, cmdline, NULL, NULL, TRUE,
                      flags == MSVCRT__P_DETACH ? DETACHED_PROCESS : 0,
                      env, NULL, &si, &pi))
   {